Capacitive sensitive touch switches are known which use a microcontroller having only a single 8-bit I/O port wherein each capacitive sensitive touch switch is directly attached to both a pin of the I/O port and to a resistor connected to the circuit's ground. The microcontroller program simultaneously measures the capacitance of all the touch switches by:
1. simultaneously writing a logic one to all eight I/O port pins, effectively charging the touch switches; PA1 2. simultaneously reading the port's eight binary logical values 64 consecutive times into the microcontroller's RAM, as the touch switches discharge through their attached resistors; PA1 3. applying a binary search algorithm to the RAM to determine at which measurement iteration each I/O port pin transitions from a logic one to a logic zero; and, PA1 4. comparing the result with earlier measurements to determine if a physical object has come in contact with a touch pad. PA1 1. widely varying capacitance of geographically distributed touch switches having widely differing trace lengths back to the microcontroller's I/O port; and, PA1 2. the speed with which a single microcontroller can process touch measurements made in the described manner. PA1 the cost of the additional I/O pins on each microcontroller for communication; PA1 the cost of developing and maintaining multiple firmware versions, possibly one for each such microcontroller; and the cost of additional firmware space to communicate among the microcontrollers. PA1 increasing the distance between each trace length and the surrounding ground plane, thereby reducing the coupling capacitance between the ground plane and the traces; PA1 staggering the starting iterations for the discharges on touch pads that discharge more quickly than others so that all pads ultimately discharge to a logic level 0 on about the same measurement iteration, thereby greatly decreasing RAM requirements; or PA1 using remote 74HCT245 transceivers to limit the capacitive dependent trace lengths since the transceivers generate digital logic at their output. Consequently, the need for extra communication software between microcontrollers is eliminated and hardware costs are reduced since a single larger microcontroller, even with outlying transceivers, is cheaper than multiple smaller microcontrollers.
These prior embodiments were limited to measuring a maximum of eight switches on a single I/O port because of two interrelated technical concerns:
Additionally, in another limitation, the described implementation requires that the 8-bit I/O port be physically dedicated to the touch measurements of the eight touch switches. The port cannot be used to make measurements on other switches, to provide communications, or to perform other common microcontroller I/O tasks.
These prior capacitive sensitive switches utilized multiple microcontrollers to avoid extra capacitance, or capacitance spread, which is engendered by long, or widely varying, trace lengths when more than eight capacitive sensitive touch pads are attached to a single microcontroller. If the capacitance measurements vary too widely, the recording of touch pad logic values for all touch pads a predetermined number of times fails since the number of readings required during the entire discharge cycle either exceeds available memory, or requires such an increase in the time delay between consecutive readings that the detection of a touch is rendered unreliable.
The extra capacitance from the trace lengths also reduces the percentage change in the switch's overall capacitance from a touch. These prior capacitive sensitive switches also utilized multiple microcontrollers to avoid the extra time required to both process and report events from multiple I/O ports attached to a single unit. Since the I/O ports must be measured sequentially, the resulting recorded data searched, and touch events reported at a rapid enough rate to provide user feedback to touch events, a single low-cost microcontroller can be overtaxed. What is desired is a faster way of measuring the data, of searching the data, or of doing both.
Thus, the described prior art required the cooperation of multiple microcontrollers when plural (more than eight) capacitive sensitive switches are used. The use of multiple microcontrollers requires increased communication, thus adding several undesirable costs:
Such prior touch switch systems also suffer from the coupling of charges between touch pads which produce erroneous indications of a touch having occurred.
In addition, such prior touch switch systems typically discharge less than all capacitive sensitive switches simultaneously. Consequently, switches that are geographically close with different discharge time schedules interact with one another unfavorably. That is, when the voltage level of one switch is kept at a constant level, nearby discharging switches provide an unnecessary and undesirable charge storage mechanism that contributes to noise and uncertainty. In a particular instance, the introduction of a foreign substance or liquid that flows slowly across multiple pads provides a mechanism for transmitting stored energy from the pad being held at a constant potential into the pad being measured, thereby adversely modifying its measurement and generating false touch events.
Other capacitive sensitive switches are known in the art as disclosed in U.S. Pat. No. 5,508,700 to Taylor, which is assigned to the assignee of the present application. According to the patent, there are a number of capacitive sensitive elements or touch pads, each of which produces an effective capacitance when touched by a physical object. A signal is supplied through switching circuits to charge a selected pad. A discharge circuit, including one or more resistors, bleeds the charge on the pad. The resulting signal from the pad, as it is discharged, is compared to a threshold value in a threshold signal circuit. The result of the comparison is used to indicate whether the pad has been touched. Taylor also discusses the reverse process of comparing a charging signal to a threshold value in a threshold signal circuit to determine when a touch has occurred.
The Taylor patent uses multiple diodes, at least one per touch pad, to select the pads for charging and discharging. Further, a microcontroller is disclosed which is used primarily as a switch control circuit to select the connections to the pads. The microcontroller may also contain a timer that responds to the signal generated by the threshold signal circuit.
Further, the microcontroller of Taylor must either utilize a large number of I/O pins or complex external switching logic. The first embodiment disclosed in Taylor uses four I/O pins to control the sensing of a single pad, an input signal, a switching signal, a discharge signal, and an interrupt signal. Because typical microprocessors such as the one disclosed in Taylor have only a small number of pins to accommodate pad connections, complicated switching circuits are usually needed to connect all of the pads to the microcontroller. For example, in a second embodiment, Taylor discloses a multiplexed pad design wherein less than one microcontroller I/O pin per pad is required. However, complicated switching and threshold detector circuits are required. Other prior art systems that require the use of multiplexer and other circuits external to a microcontroller to overcome I/O pin limitations are U.S. Pat. Nos. 4,698,460; 4,698,461; 4,707,845; 4,733,222; 4,772,874; 5,012,124; 5,053,757; 5,130,661; 5,270,710; 5,386,219; 5,463,388; 5,469,364; and 5,594,222.
A further limitation is that the circuit disclosed in Taylor is generally operable for only a single type of application, since the complex switching logic requires that I/O pins be dedicated to special purpose tasks. I/O pins thus must be reassigned, and the microcontroller firmware must be redesigned before another type of application may be accommodated.
Other known capacitive sensitive switches (such as those disclosed in U.S. Pat. Nos. 4,707,845; 5,012,124; 5,053,757; 5,072,427; 5,270,710; 5,386,219; 5,463,388; 5,565,658; and 5,594,222) use a tuned circuit or a high-frequency oscillator. A physical object touching a pad changes the oscillator frequency, and the change in frequency is detected as a touch. Oscillator circuits usually require expensive discrete components.
Still other capacitive sensitive switches use a change in voltage to detect a touch. These circuits use an external comparator in a detection circuit such as that disclosed in U.S. Pat. No. 5,289,521 to compare the voltage from the touch pad with a voltage from a reference circuit. Both the comparator and the reference circuit add considerable expense to the overall cost of a circuit. The reference circuit also becomes unreliable as its components age or environmental conditions change.
Further capacitive sensitive elements such as those disclosed in U.S. Pat. Nos. 4,698,460; 4,698,461; 4,707,845; 4,733,222; 4,860,232; 5,012,124; 5,053,757; 5,130,661; 5,270,710; 5,386,219; 5,459,529; and 5,586,042 require the use of analog-to-digital or digital-to-analog converters. Such converters are expensive when used as components external to a microcontroller, and raise the cost of the microcontroller when integrated as part of the microcontroller.
Other capacitive sensitive switches may use touch pads comprised of two or more plate elements, which increases cost and complicates circuit routing.
Many prior art switches are also susceptible to random noise which affects the reliability of the switch. Such noise is difficult to eliminate when only one switch measurement can be made during a given time period.
Most known switches do not permit the simultaneous measurement of signal levels for plural touch pads. Of the few switches that do permit simultaneous measurement, most employ complex multiplexer circuitry and duplicate circuit elements external to the microcontroller. Such circuitry often requires active components or is subject to current leakage, particularly at high temperatures. Current leakage affects measurement accuracy. In addition, duplication of circuitry raises the cost of the switch system.
Some prior touch switch systems use microcontrollers to sense and record touch pad logic levels a predetermined number of times, and use a binary search algorithm to search all recorded RAM locations to determine when each touch pad has undergone a logic transition. For an eight bit I/O port, 64 bytes of RAM memory are typically required, and all such memory locations are thereafter accessed to determine when each I/O pin has transitioned from one logic level to another.
The present invention utilizes a single microcontroller, with multiple I/O ports, to support plural capacitive sensitive switches. Further, the trace length dependence has been reduced through three methods:
Staggering the starting iterations for the capacitive sensitive touch pad discharge has the added benefit of noise reduction since each capacitive element is more likely to experience the same environmental factors during the terminal, and most critical phase, of its decay--a transition in logic states.
While staggered discharge start times and the use of 74HCTZ45 transceivers are mutually exclusive on an I/O port, they do help accomplish the same goal of narrowing the iteration interval over which the I/O port's pins undergo logic level transitions.
The present invention also addresses the problem of potentially large RAM requirements by developing a measurement method that records only the changes in the recorded logic levels at a port and their respective transition times. This process is particularly well suited to faster microcontrollers and, for an eight bit I/O port, requires at most 16 bytes of RAM rather than the 64 bytes of RAM as occurs in some prior systems. Referring to Table I below, the method of recording only logic transitions in an I/O port and the time of their occurrence would result in a table similar to the following:
TABLE I ______________________________________ Iteration Binary Port Value ______________________________________ 7 11111011 9 11011011 11 11011001 12 01011001 14 01000001 ______________________________________
This recording method of the present invention also accommodates the extraction of the actual transition values more quickly than in the prior art. Further, in the present invention, staggering the starts of the discharge cycles for the affected pads decreases the size of the RAM to be searched, thus speeding the search cycle which otherwise would account for a majority of the touch switch process time.
The use of the 74HCT245 transceivers also allows for the use of a single 8-bit I/O port to perform measurements sequentially on multiple groups of touch switches or for other I/O operations.
Rather than using square cornered touch pads which may increase the likelihood of charge coupling between touch pads, the present invention uses touch pads with rounded corners to reduce the coupling between touch pads. The present invention further utilizes a single conductive plate as the capacitive element, and adds a ground plane to provide further noise reduction. In addition, guard rings are employed to inhibit responses to spurious inputs.
Finally, the present invention extends the above innovations to include interdigitated capacitive sensitive elements whose simultaneous readings can be used to assign one of a plurality of values to a single touch while reducing the effects of noise.
It would thus be desirable to provide a low cost capacitive sensitive switch system which requires fewer circuit components, which is less susceptible to random noise or current leakage problems, which has reduced RAM requirements, and is more reliable than prior touch switch systems, which may be easily modified to adapt to changing environmental conditions and application requirements, and which will accommodate the simultaneous measurement of signal levels of plural touch pads or capacitive elements.